1. Field of the Invention
The present invention relates generally to a charge pump-type booster circuit. More particularly, the invention relates to a DC/DC converter circuit which converts a supplied direct current voltage into an arbitrary level of direct current voltage, and further particularly to a charge pump-type booster circuit generating a higher voltage from a single supply power source.
2. Description of the Related Art
A charge-pump type booster circuit constituted of one or more electronic switches, such as transistors and so forth, and one or more capacitors, is a circuit for boosting an externally supplied voltage to a required higher voltage. This circuit can be made compact and lightweight by integrating the electronic switch with semiconductor transistor, thin film transistor or the like. Therefore, the charge-pump type booster circuit is employed in portable equipments, such as cellular phone, personal computer and so forth.
One example of this kind of technology has been disclosed in Japanese Unexamined Patent Publication No. 2000-236658 and Japanese Unexamined Patent Publication No. Heisei 9-191639. FIG. 10 is a circuit diagram of one example of the conventional triple booster circuit already shown in FIG. 6 of Japanese Unexamined Patent Publication No. 2000-236658 and in FIG. 3 of Japanese Unexamined Patent Publication No. Heisei 9-191639.
This circuit is constructed with at least four charge switches, two charge capacitors, three boosting switches and an output capacitor holding boosted voltage and constantly grounded at one side.
A charge switch 11 connects a terminal 72 of an input power source 1 and a terminal 75 of a charge capacitor 61. A charge switch 12 connects a terminal 74 of the charge capacitor 61 and a grounding point 71. A charge switch 13 connects a terminal 72 of an input power source 1 and a terminal 79 of a charge capacitor 62. A charge switch 14 connects a terminal 78 of the charge capacitor 62 and a grounding point 71.
The boosting switch 21 connects the terminal 72 of the input power source 1 and the terminal 74 of the charge capacitor 61. The boosting switch 22 connects the terminal 75 of the charge capacitor 61 and the terminal 78 of the charge capacitor 62. The boosting switch 23 connects the terminal 79 of the charge capacitor 62 and a terminal 77 of an output capacitor 51.
Next, a timing chart of operation of the switch in FIG. 10 is shown in FIG. 11. When the charge switches 11, 12, 13 and 14 become conductive (ON) and the boosting switches 21, 22 and 23 become non-conductive (OFF), the charge capacitors 61 and 62 are connected to input power source 1 to be charged with an input voltage. Next, when the boosting switches 21, 22 and 23 become conductive (ON) and the charge switches 11, 12, 13 and 14 become non-conductive (OFF), the input power source 1 and the charge capacitors 61 and 62 are connected in series. Then, the output capacitor 51 is charged at triple of the input voltage, and triple boosted voltage is supplied to a load 52.
When a voltage is supplied to a display device built-in a portable equipment, it becomes necessary to generate a plurality of voltages for data line driving circuit, gate line driving circuit and so forth from a single power source. As an example of conventional construction, a circuit simultaneously supplying twice boosted voltage and three times boosted voltage is shown in FIG. 12. Difference to FIG. 10 is that the boosting switch 22, the output capacitor 53 and the load 54 are added for supplying twice boosted voltage to the load.
The boosting switch 22 connects the terminal 75 of the charge capacitor 61 and the terminal 76 of the output capacitor 53. The switch of this circuit operates at a timing shown in FIG. 11. This circuit is constructed at least with four charge switches, two charge capacitors, four boosting switches and two output capacitors holding boosted voltages.
When the electronic switch is constructed with MOS (Metal-Oxide Semiconductor) transistor, it becomes necessary to form the switch with a huge size of transistor in order to lower a resistance upon conduction of the switch, namely on resistance. Therefore, a layout area is increased according to increasing of number of switches. On the other hand, when the capacitors used for holding the charge and output are formed by external parts of the integrated circuit, such as ceramic capacitor or the like, increasing of number of capacitors serves as hindrance of decreasing of size and weight of the power source circuit. On the other hand, upon building in the capacitor in the integrated circuit, it results in increasing of layout area of the circuit.
When the booster circuit is applied to the portable equipment, decreasing of size and weight and decreasing of power consumption are demanded. In the charge pump-type booster circuit, it is effective to reduce number of capacitors for reducing weight and area. Also, reducing of number of switches forming the circuit may result in reduction of the area.